Finite Element Based Fracture Assessment of HP/ttT Subsea Pipelines

The current demand for hydrocarbon products and the recent advances in technology have meant that reservoirs previously thought to be uneconomical are now being considered for development. As part of the recent advances in technology, Pipe-In-Pipe systems are being considered and adopted in High Pressure / High Temperature (HP/HT) pipeline in order to provide the required thermal insulation which cannot be achieved with conventional single pipe insulation coatings. The structural force transfer mechanism of Pipe-ln-Pipe systems during operation is such that the outer pipe will be subjected to very high tensile load that balance part of the axial compressive forces induced in the HP/HT inner pipe (Sriskandarajah 1998). In addition, pressure and temperature fluctuations during operation introduce tensile stress variations in the outer pipe which increase the risk of fatigue and fracture induced failure modes. Also, pipeline installation loads, and planned or un-planned shut downs and restarts, add very large tensile stress variations to the outer pipe, and thus contribute to fatigue damage accumulation. Structural failure due to fracture is mainly due to tensile loads acting through stress concentration locations, such as those at the outer pipe field joint welds, where weld defects could also Field joint weld details represent one of the weak links in the Pipe-In-Pipe systems and need to be scrutinised in detail design stage. Due to practical difficulties associated with the joints during offshore installation where clearly inner pipe segments need to be welded prior to the outer pipes, "Half Split Sleeve" or "Sliding Collar" outer pipe field joint arrangements have to be devised to complete the final connections. In the Sliding Collar arrangement, the collar is attached to the sleeve pipe via a single fillet weld.